Protein kinases are involved in regulation of practically all processes that are central to the growth, development, and homeostasis of eukaryotic cells. In addition, some protein kinases have an important role in oncogenesis and tumor progression and several kinase inhibitors are now approved for the treatment of cancer (D. J. Matthews and M. E. Gerritsen: Targeting protein kinases for cancer therapy, Wiley, 2010).
Examples of kinase inhibitors that are used in modern oncology include: imatinib (treatment of CML); dasatinib (CML with resistance to prior treatment, including imatinib); nilotinib (CML); bosutinib (CML); gefitinib (non-small cell lung cancer); erlotinib (non-small cell lung cancer and pancreatic cancer); lapatinib (breast cancer); sorafenib (metastatic renal cell carcinoma, hepatocellular cancer); vandetanib (metastatic medullary thyroid cancer); vemurafenib (inoperable or metastatic melanoma); crizotinib (non-small cell lung cancer); sunitinib (metastatic renal cell carcinoma, gastrointestinal stromal tumor that is not responding to imatinib, or pancreatic neuroendocrine tumors); pazopanib (renal cell carcinoma and advanced soft tissue sarcoma); regorafenib (metastatic colorectal cancer); cabozantinib (metastatic medullary thyroid cancer); dabrafenib (BRAF V600E mutation-positive advanced melanoma); and trametinib (in combination with dabrafenib for the treatment of BRAF V600E/K-mutant metastatic melanoma).
Various kinases are regarded as good targets for pharmacological inhibition in order to treat proliferative and/or neurodegenerative diseases. Biological and potential therapeutic significance of some selected kinases is briefly summarized below.
The regulation of splice site usage provides a versatile mechanism for controlling gene expression and for the generation of proteome diversity, playing an essential role in many biological processes. The importance of alternative splicing is further illustrated by the increasing number of human diseases that have been attributed to mis-splicing events. Appropriate spatial and temporal generation of splicing variants demands that alternative splicing be subjected to extensive regulation, similar to transcriptional control. The CLK (Cdc2-like kinase) family has been implicated in splicing control (Experimental Cell Research 1998, 241, 300.). Pharmacological inhibition of CLK1/Sty results in blockage of SF2/ASF-dependent splicing of beta-globin pre-mRNA in vitro by suppression of CLK-mediated phosphorylation. It also suppresses dissociation of nuclear speckles as well as CLK1/Sty-dependent alternative splicing in mammalian cells and was shown to rescue the embryonic defects induced by excessive CLK activity in Xenopus (Journal of Biological Chemistry 2004, 279, 24246.).
Alternative mRNA splicing is a mechanism to regulate protein isoform expression and is regulated by alternative splicing factors. The alternative splicing factor 45 (SPF45) is overexpressed in cancer and its overexpression enhances two processes that are important for metastasis, i.e. cell migration and invasion, dependent on biochemical regulation by CLK1 (Nucleic Acids Research 2013, 41, 4949.). CLK1 phosphorylates SPF45 on eight serine residues. CLK1 expression enhances, whereas CLK1 inhibition reduces, SPF45-induced exon 6 exclusion from Fas mRNA. Inhibition of CLK1 increases SPF45 degradation through a proteasome-dependent pathway. In addition, small-molecule inhibitors of specific CLKs can suppress HIV-1 gene expression and replication (Retrovirology 2011, 8, 47.), which could be used in concert with current drug combinations to achieve more efficient treatment of the infection. Inhibition of CLK1 can be applicable in the treatment of Alzmeimer's disease (Current Drug Targets 2014, 15, 539.).
DYRK (dual specificity tyrosine phosphorylation-regulated kinase) family enzymes are essential components of important signaling cascades in the pathophysiology of cancer and Alzheimer's disease and their biological expression levels regulate key signaling processes in these diseases. In particular, DYRK2 is over-expressed in adenocarcinomas of the esophagus and lung (Cancer Research 2003, 63, 4136.) and DYRK1A in glioblastoma where its inhibition compromised tumors'survival and produced a profound decrease in tumor burden (Journal of Clinical Investigation 2013, 123, 2475.). DYRK1B activation that is induced by microtubule damage triggers microtubule stabilization and promotes the mitochondrial translocation of p21Cip1/waf1 to suppress apoptosis. Its inhibition caused reduced viability of cancer cells (ACS Chemical Biology 2014, 9, 731.). Correspondingly, it has been understood that inhibition of DYRK kinases alone or in combination with other chemotherapeutic drugs may have tumor suppression effect and the enzymes are therefore appropriate targets for pharmacological inhibition (Bioorgank & Medicinal Chemistry Letters 2013, 23, 6610; Medicinal Chemistry Research 2014, 23, 1925.).
In addition, DYRK kinases are also over-expressed in neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and Pick disease (Neurobiology of Disease 2005, 20, 392; Cellular and Molecular Life Sciences 2009, 66, 3235.).
HIPK2 (homeodomain-interacting protein kinase) is a tumor suppressor and functions as an evolutionary conserved regulator of signaling and gene expression. This kinase regulates a vast array of biological processes that range from the DNA damage response and apoptosis to hypoxia signaling and cell proliferation. Recent studies showed the tight control of HIPK2 by hierarchically occurring posttranslational modifications such as phosphorylation, small ubiquitin-like modifier modification, acetylation, and ubiquitination. Dysregulation of HIPK2 can result in increased proliferation of cell populations as it occurs in cancer or fibrosis. Inappropriate expression, modification, or localization of HIPK2 can be a driver for these proliferative diseases (Journal of Molecular Medicine 2013, 91, 1051.).
FMS-like tyrosine kinase 3 (FLT3), a receptor tyrosine kinase (RTK), is a membrane-bound receptor with an intrinsic tyrosine kinase domain. Its activation regulates a number of cellular processes (e.g. phospholipid metabolism, transcription, proliferation, and apoptosis), and through these processes, FLT3 activation plays a critical role in governing normal hematopoiesis and cellular growth Expression of FLT3 has been evaluated in hematologic malignancies. The majority of B-cell acute lymphocytic leukemia (ALL) and acute myeloid leukemia (AML) blasts (>90%) express FLT3 at various levels (Clinical Cancer Research 2009, 15, 4263.). Overexpression or/and activating mutation of FLT3 kinase play a major driving role in the pathogenesis of acute myeloid leukemia (AML). Hence, pharmacologic inhibitors of FLT3 are of therapeutic potential for AML treatment (Oncologist 2011, 16, 1162; PLoS One 2014, 9, e83160/1; Leukemia Lymphoma 2014, 55, 243.).
Tropomyosin-related kinase (TRK) is a family of three RTKs (TRK-A, TRK-B, TRK-C) regulating several signaling pathways that are important for survival and differentiation of neurons. TRK-A regulates proliferation and is important for development and maturation of the nervous system, promotes survival of cells from death. Point mutations, deletions and chromosomal rearrangements cause ligand-independent receptor dimerization and activation of TRK-A. In mutated version of TRK, abnormal function will render cells unable to undergo differentiation in response to ligand in their microenvironment, so they would continue to grow when they should differentiate, and survive when they should die. Activated TRK-A oncogenes have been associated with several human malignancies, e.g., breast, colon, prostate, thyroid carcinomas and AML (Cell Cycle 2005, 4, 8; Cancer Letters 2006, 232, 90.). In addition, inhibition of TRK can be relevant for the treatment of inflammation (PLoS One 2013, 8, e83380.) and pain (Expert Opinion on Therapeutic Patients 2009, 19, 305.).
In summary, there is a need for inhibitors of different protein kinases in order to treat or prevent disease states associated with abnormal regulation of the kinases-mediated biological processes.